The life cycle and body plan of flowering plants differs from that of invertebrate and vertebrate animal. During animal embryogenesis, a smaller version of the adult is generated with a full complement of nearly complete organs and germ line. Plant embryos establish only the basic root-shoot axis, a cotyledon storage organ(s), and stem cell populations, called the root and shoot apical meristems, that continuously generate new organs throughout post-embryonic development. In Arabidopsis, the seedling shoot apical meristem produces the vegetative stem and leaves of the plant. In response to genetic and environmental information, the shoot apical meristem undergoes a dramatic transition, enters the reproductive phase, and becomes the primary inflorescence meristem. See, e.g., M. Koornneef, et al., Annu. Rev. of Plant Physiol. Plant Mol. Biol. 49, 345-370 (1998); Y. Y. Levy & C. Dean, Plant Cell 10, 1973-1998 (1998); M. Pineiro & G. Coupland, Plant Physiol 117, 1-8 (1998); T. Araki, Curr Opinion in Plant Biology 4, 63-68 (2001).
During the early reproductive phase, the inflorescence meristem generates secondary inflorescence meristems subtended by specialized cauline leaves. During the late reproductive phase, solitary flowers are formed composed of sepals, petals, stamens bearing male gametophytes, and carpels bearing female gametophytes. See E. A. Schultz, et al., Development 119, 745-765 (1993). Fertilization of the egg by one sperm produces the embryo, whereas fertilization of the adjacent central cell by a second sperm produces the endosperm, an organ that nutritionally supports the development of the embryo with the seed.
Polycomb proteins are essential regulators of animal embryogenesis. See V. Pirrotta, Cell 93, 333-336 (1998). They have evolved to control different processes depending upon the distinct life cycle and body plan of the organism. In Drosophila and mouse, polycomb proteins are responsible for the long-term repression of homeotic genes. This repression ensures that each body segment bears a distinct array of homeotic transcription factors which determine its identity. During Caenorhabditis embryogenesis, polycomb proteins are required to establish the germ line. The WD-motif polycomb proteins, as exemplified by the related Drosophila ESC (S. S. Sathe, et al., Mech. Dev. 52, 77-87 (1995)), mouse EED (A. Schumacher, et al. Nature 383, 250-253 (1996)), and Caenorhabditis MES6 proteins (I. Korf, et al., Development 125, 2469-2478 (1998)), play an essential role in polycomb complex assembly. The WD repeats form surface loops that are used as scaffolds for the generation of protein complexes including additional polycomb proteins (e.g., SET-domain polycombs) and histone deacetylase that remodel chromatin and repress gene transcription (J. van der Vlag, et al., Nat. Genet. 23, 474-478 (1999); F. Tie,et al., Development 128, 275-266 (2001)).
Polycomb proteins are also important for plant reproduction (Pirrotta, supra). Arabidopsis FIE (FERTILIZATION INDEPENDENT ENDOSPERM) is a WD-motif polycomb related to ESC, EED, and MES6 (N. Ohad, et al., Plant Cell 11, 407-415 (1999)). Prior to fertilization, FIE prevents the premature replication of the central cell to form an endosperm, and after fertilization, the maternal FIE allele is essential for seed viability (R. Vinkenoog, et al., Plant Cell 12, 2271-2282 (2000); N. Ohad, et al., Proc. Natl. Acad. Sci. USA 93, 5319-5324 (1996)). Because maternal mutant fie alleles are not transmitted to the next generation, it has not been possible to generate and examine the phenotype of homozygous fie mutant seedlings or plants (Ohad, 1996, supra; Ohad, 1999, supra; A. M. Chaudhury, et al., Proc. Natl. Acad. Sci. USA 94, 4223-4228 (1997)). The Arabadopsis polycomb gene, MEDEA (MEA), contains a SET domain with homology to the Drosophila polycomb gene enhancer of zeste (E(z)) (Grossniklaus, et al., Science 280, 446-450 (1998). The MEA gene product interacts with FIE.
Thus, the extent that polycomb gene complexes regulate post-embryonic plant development is not known. The present invention addresses this and other problems.